The problem was that the SPI, wich writes to SD-card, shared MISO/MOSI-pins with the I2C-bus, reading from the sensors. The only thing I did was to put an SPI.end() in the beginning of the ISR -routine (wich uses I2C) and an SPI.begin() in the end of the ISR-routine.
The ISR uses the Ticker class, which I think is asynchronous and is therefor less restrictive on the time it takes for the ISR to finish, than synchronous ISRs (see link). My ISR-routine takes about 820 us.
Thanks for the help guys!